This is a continuing study of marrow grafting of support for the lethally irradiated animal and of the associated immunological problems. Experimental subjects will be the dog. Where appropriate, clinical studies in patients will be carried out. Specific problems to be studied include the following: 1. Treatment and prevention of secondary disease. 2. Studies on the effect of prior transfusions on subsequent marrow grafts. 3. Studies on the nature of tolerance in canine marrow chimeras and on the use of "blocking factor" in transplantation studies. 4. Studies on the immune status in canine marrow chimeras. 5. Long-term observation of canine radiation chimeras. 6. Studies on combinations of radiation and chemotherapeutic agents in preparing recipients for marrow grafts. 7. Studies on canine histocompatibility typing and marrow grafting. 8. Studies on the effect of the radiation exposure rate in allogeneic marrow grafting. 9. Studies on marrow grafting and secondary disease in animals. 10. Studies on extracorporeal irradiation of the blood in man.